1. Field of the Invention
This invention relates in general to a ballast circuit for a discharge tube and in particular to a high frequency electronic power supply and ballast circuit for a fluorescent tube.
2. Description of the Prior Art
High speed switching solid-state electronic power supplies and ballasts can be designed to be smaller, lighter and more efficient than conventional core and coil transformers and ballasts designed for the same end use. They work by converting a.c. to d.c., then switching the d.c. at speeds much higher than the original a.c.. These highers speeds, between 30 and 30,000 times the incoming frequency, permit a drastic reduction in the size and weight of the magnetic components such as inductors and transformers.
There are numerous examples of prior art electronic ballast circuits for discharge tubes. For example, U.S. Pat. No. 4,370,600 issued Jan. 25, 1987, to Zansky concerns a two-wire electronic dimming ballast for fluorescent lamps. Filaments of the lamps are supplied by auxiliary coils of an inductor connected in series with a capacitor in a series resonant bridge inverter. The power to the lamps is supplied via such capacitor.
U.S. Pat. No. 4,392,087 issued July 5, 1983 to Zansky concerns a two-wire electronic dimming ballast circuit for gaseous discharge lamps which illustrates at FIG. 6 a positive feedback self-oscillating circuit for switching rectified d.c. voltage to the primary of a transformer. A tuning capacitor is connected across the main secondary winding. The system provides a sine wave input at the natural oscillating frequency of a half-bridge inverter circuit as determined by the leakage inductance of the main secondary winding and the tuning capacitor. FIGS. 7 and 8 of the '087 illustrate alternative embodiments of the circuit of FIG. 6.
U.S. Pat. No. 4,045,711 issued Aug. 30, 1977 to Pitel illustrates another tuned oscillator ballast circuit, but a series inductive winding and capacitor are provided in the input of a transformer which are series resonant at the oscillating frequency.
While the electronic ballast circuits described above as well as others have certain advantages, their effectiveness in striking and thereafter regulating the current applied to a discharge tube such as a fluorescent bulb has been less than ideal. For example, such circuits have not been ideally effective in reducing RFI/EMI generated by the HID or fluorescent bulb.